The present invention relates to fuel cells and, more particularly to a system and method for improving fuel cell behavior during shutdown, which improves startup after exposure to sub-freezing conditions.
Electrochemical fuel cell assemblies such as proten exchange membrane (PEM) fuel cells are known for their ability to produce electricity and a subsequent reaction product through the reaction of a fuel being provided to an anode and oxidant being provided to a cathode, thereby generating a potential between these electrodes. Such fuel cell assemblies are very useful and sought after due to their high efficiency, particularly as compared to internal combustion fuel systems and the like. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction byproducts that are produced, such as water. In order to control the temperature within the fuel cell assembly, a coolant is provided to the fuel cell assembly, and this coolant may also typically be water. Thus, water circulates through the fuel cell assembly during operation of same.
One particularly attractive use for fuel cell assemblies is in vehicular applications. However, a critical problem in connection with such use is the sensitivity of the fuel cell assembly to sub-freezing temperatures, and the water circulating through a fuel cell assembly is particularly susceptible to freezing in the fuel cell and creating serious problems for subsequent startup.
A number of solutions to this problem have been attempted, mostly including methods for removing water from the fuel cell assembly before such water reaches a freezing temperature. Despite these efforts, the need remains for a fuel cell system which allows for rapid startup after sub-freezing conditions, without substantially increasing the size, cost or startup time of the fuel cell.
It is therefore the primary object of the present invention to provide such a fuel cell system.
In accordance with the present invention, the foregoing objects and advantages have been readily attained.
According to the invention, a PEM fuel cell system is provided, which comprises a plurality of PEM fuel cells arranged in a stack having two opposed, outwardly facing end surfaces; pressure plates positioned relative to said end surfaces for securing said PEM fuel cells in said stack; and spacer members between said end surfaces and said pressure plates for thermally insulating said end surfaces from said pressure plates.
The spacer members may advantageously be wettable, and preferably porous and/or permeable to water, and further preferably have a thermal conductivity which is less than the pressure plates, whereby (1) end surfaces of the fuel cell stack are thermally insulated from the pressure plates, and (2) water which migrates to end surfaces is captured by the spacer member.
In further accordance with the present invention, a method is provided for shutting down a PEM fuel cell system comprising a plurality of PEM fuel cells arranged in a stack having two opposed outwardly facing end surfaces and pressure plates positioned relative to said end surfaces for securing said PEM fuel cells in said stack, wherein said method comprises the steps of positioning spacer members between said end surfaces and said pressure plates for thermally insulating said end surfaces from said pressure plates; and cooling said PEM fuel cell system whereby said spacer members thermally insulate said end surfaces from said pressure plates thereby reducing migration of water toward said end surfaces.